This invention relates to polysaccharide polymers. In particular, it relates to xanthan-based polysaccharide polymers, defined herein as polymers structurally similar to xanthan gum and produced by components of the xanthan biosynthetic pathway, including those xanthan-based polymers modified so that the outer mannose can be specifically acetylated but not pyruvylated, pyruvylated but not acetylated, or unmodified while the inner mannose can be independently controlled to be acetylated or unmodified.
Xanthan gum is produced by bacteria of the genus Xanthomonas, in particular by microorganisms of the species X. campestris. Xanthan gum is a widely used product due to its unusual physical properties, i.e., its extremely high specific viscosity and its pseudoplasticity. It is commonly used in foods as a thickening agent and in secondary or tertiary oil recovery as a mobility control and profile modification agent, as well as in petroleum drilling fluids.
Chemically, xanthan gum is an anionic heteropolysaccharide. The repeating unit of the polymer is a pentamer composed of five sugar moieties, specifically two glucose, one glucuronic acid and two mannose moieties. These sugar residues are arranged such that the glucose moieties form the backbone of the polymer chain, with side chains of mannose-glucuronic acid-mannose residues generally extending from alternate glucose moieties. Usually, this basic structure is specifically acetylated and pyruvylated, as described, for example, by Janson, P. E., Kenne, L., and Lindberg, B., in Carbohydrate Research, 45:275-282 (1975) and Melton, L. D., Minot, L., Rees, D. A., and Sanderson, G. R., in Carbohydrate Research, 46:245-257 (1976), each of which is specifically incorporated herein by reference. The extent of acetylation and pyruvylation is known to vary. The structure of xanthan gum is depicted in formula I below: ##STR1##
In spite of the broad utility of naturally-occurring xanthan gum, there are some situations where its physical properties become limiting. In particular, in secondary or tertiary oil recovery it is not uncommon for the temperature of the oil bearing reservoir and the salt concentrations in the reservoir brine to be higher than are optimal for xanthan solutions. When these conditions occur, xanthan can precipitate, flocculate and/or lose its viscosity. Therefore, new viscosifying products which perform well at various conditions encountered during oil recovery, such as high temperature and high salt concentrations would be desirable.
The present invention discloses a family of xanthan-based polysaccharides having improved properties relative to naturally-occurring xanthan gum. Modifications of xanthan gum have been previously described. For example, Bradshaw et al. (Carbohydrate Polymers, 3:23-38 (1983)) describe methods for preparing chemically-modified xanthan gum which is deacetylated or depyruvylated. Various means of chemically deacetylating xanthan gum produced by Xanthomonas campestris also are described in U.S. Pat. Nos. 3,000,790 and 3,054,689. To date, the predominant method utilized for these deacetylation processes has been chemical removal of the acetate moieties from normally acetylated xanthan gum. It has been found that chemical processes for deacetylating xanthan gums can result in a number of undesirable side effects and may cause hydrolysis of the glycosidic backbone, resulting in an irreversible change in the conformation of the molecule and lowered molecular weight.
Some of the rheological properties of deacetylated xanthan in aqueous media are known. See, e.g., Tako and Nakamura, Agric. Biol. Chem. 48:2987-2993 (1984) and U.S. Pat. Nos. 3,000,790 and 3,054,689. Also, a method of increasing the viscosity of an aqueous solution using a deacetylated polysaccharide is described in U.S. Pat. No. 3,096,293. Thus, a method for obtaining non-acetylated xanthan which does not cause untoward side effects has been sought.
Xanthan gum can be chemically depyruvylated as well, as described by Holzwarth and Ogletree in Carbo. Res. 76:277-280 (1979). This chemical method of depyruvylation also can alter the xanthan polymeric unit and/or cause hydrolysis of the glycosidic backbone. While a strain of X. campestris has been described in U.S. Pat. No. 4,296,203 which produces non-pyruvylated xanthan gum, this non-pyruvylated gum was either fully acetylated or deacetylated using chemical means.
Additionally, the extent of acetylation of the internal mannose on the xanthan side chain and the extent of the pyruvylation of the terminal mannose may vary. The present inventors believe that a fully acetylated and/or fully pyruvylated xanthan will have improved rheological properties for certain oil recovery purposes.
Moreover, the present inventors have identified polysaccharides which are based on alterations of the normal xanthan pentamer building block. These polymers exhibit improved rheological properties over normal xanthan gum with respect to shear rate, their ability to tolerate salinity and their response to temperature as it affects their viscosifying properties. These altered polysaccharides include the polytrimer which is depicted below and the non-acetylated polytetramer. ##STR2##
These polysaccharides also include the acetylated and non-acetylated polytrimer described by Vanderslice et al. in U.S. Pat. No. 4,713,449 entitled "A Polysaccharide Polymer Made By Xanthomonas," issued Dec. 15, 1987, which is specifically incorporated herein by reference.
An object of the present invention is to provide a family of polysaccharide polymers which are better viscosifiers of water than naturally-occurring xanthan gum. Another object of the present invention is to provide a family of polysaccharide polymers having improved rheological properties over naturally-occurring xanthan gum at elevated temperatures and/or in the presence of salts and which members possess other desired properties.
An object of the present invention is to provide a family of polysaccharide xanthan polymers in which the inner mannose is acetylated or unmodified while the outer mannose is acetylated, pyruvylated or unmodified.
It is also an object of the present invention to provide an in vitro method for obtaining these products and microorganisms having the ability to produce members of this family of polysaccharide polymers in vivo. A further object of the present invention is to provide processes for preparing members of this family of polysaccharides by aerobically fermenting microorganisms having the ability to produce the various polysaccharide polymers.
It is also an object of the present invention to provide a process for preparing members of this family of polysaccharides by aerobically fermenting microorganisms containing chromosomal mutations which give these microorganisms the ability to produce the various polysaccharide polymers.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.